A digital magnetic compass is shown in copending U.S. patent application Ser. No. 284,362, filed Aug. 28, 1972, wherein compass headings are optically coded on a plurality of tracks of a compass disc which is electro-optically sensed to provide electrical output signals representative of compass heading. It is useful to employ the minimum electrical energy necessary to energize the light source associated with the compass disc sensing apparatus, since such energy for providing illumination is a major portion of that required for compass operation.